BOD Calculator

Biochemical oxygen demand measures the oxygen microorganisms consume while oxidizing degradable organic matter in a water sample, making it a proxy for organic pollution load rather than a measurement of any single compound. The test is empirical: a diluted sample is sealed in a 300 milliliter bottle, incubated in the dark at 20 degrees C, and the dissolved oxygen drop over five days is multiplied by the dilution factor to give BOD5 in milligrams per liter. Dilution is required because oxygen saturation at 20 degrees C is only about 9.1 mg/L; Standard Methods 5210B treats a bottle as valid only if depletion reaches at least 2.0 mg/L and at least 1.0 mg/L of oxygen remains at the end. Oxidation follows approximately first-order kinetics, so the demand exerted by time t equals ultimate demand times one minus the exponential of minus k times t. A deoxygenation constant of 0.23 per day on a natural log basis is the conventional default for domestic wastewater, which places BOD5 at roughly 68 percent of ultimate BOD. Raw municipal sewage typically runs 110 to 400 mg/L, well-treated secondary effluent under 30 mg/L, and unpolluted rivers under 2 mg/L. Nitrifying bacteria exert a second, nitrogenous demand after about five to eight days unless an inhibitor is added, and toxic or metal-laden samples give falsely low results.

The five-day incubation is an accident of British geography. The Royal Commission on Sewage Disposal, which sat in the United Kingdom from 1898 to 1915, needed a standard measure of the polluting strength of sewage and chose oxygen absorbed over five days on the reasoning that no major British river took longer than five days to reach the sea. Its 1912 report paired the test with an effluent limit of 20 mg/L BOD and 30 mg/L suspended solids, the origin of the still-quoted 20:30 standard. The original incubation temperature was 18.3 degrees C, the assumed British summer river temperature quoted as 65 degrees Fahrenheit; American practice later standardized on 20 degrees C. The kinetic interpretation came from Harold Streeter and Earle Phelps, whose 1925 US Public Health Service study of the Ohio River treated deoxygenation and reaeration as competing first-order processes and produced the dissolved oxygen sag curve still used in waste load allocation. Dilution water preparation, seeding, and suppression of nitrification using allylthiourea or 2-chloro-6-trichloromethylpyridine were codified across successive editions of Standard Methods for the Examination of Water and Wastewater, where the procedure carries the designation 5210B.